In the past, the direct view storage tube has become a familiar component in processing systems incorporating graphic display terminals. This popularity is, of course, because the cost incured to provide excellent image presentations is quite low. This is possible because as fully explained in U.S. Pat. No. 3,293,473 to Robert H. Anderson, the direct view storage tube can store images directly on its screen without refreshing. This has allowed the use of low speed deflection amplifiers thereby permitting the manufacture of the graphic terminal at substantially less cost than conventional terminals. Additionally, because data can be continuously added to a direct view storage tube, very complex images can be displayed.
A minor disadvantage with processing systems utilizing these above-mentioned direct view storage tubes, however, is that the stored images are static. Information may be continually added, but to remove a single element of information requires that the entire image be erased and all elements except the deleted item be rewritten. As is well known, this involves an erase procedue in which the entire screen is illuminated briefly and requires a varying amount of rewrite time depending on the data source and deflection system used. Since systems utilizing such storage tubes usually do not have any local memory associated therewith and their deflection systems are limited, this rewrite time may range up to tens of seconds. Clearly such delays are distracting to a user and dynamic displays with moving or rotating objects are seldom attempted. Dynamic images, on the other hand, are more easily generated on the more costly conventional cathode ray tubes having local memories and operated to write the image 30 to 60 times per second, as is well known.
As a result of the distinct performance difference associated with dynamic images, the user is affixed to a system utilizing the high capacity direct view storage tubes for static images or to a system utilizing a refreshed cathode ray tube for providing dynamic image capability.
In reality, there has always been a method in which dynamic capability may be incorporated into the direct view storage tubes. Such method is the so called .cent.Write-Through" which is the operation of a direct view storage tube to produce a non-stored image during storage of another image as fully explained in U.S. Pat. No. 3,430,093 to C. N. Winningstad. Unfortunately, because of a desire to keep costs to a minimum, this mode of operation has never been utlized to its fullest potential, a clear disadvantage.